1. Field of the Invention
The present invention relates to an image processing apparatus, which is used to execute, for example, edge processing of an image object, and a control method of an image forming apparatus.
2. Description of the Related Art
Some electrophotographic image forming apparatuses include a function of executing trapping processing for slightly overlapping the boundaries of objects such as images, graphics, characters, and the like on different color plates, and edge smoothing processing for smoothing the edge of an object after dithering. The trapping processing is executed to prevent a blank (called a boundary gap) in the boundaries between objects formed by different color plates, by expanding the edge of at least one of the objects whose boundaries contact each other by a predetermined amount. Then, the objects overlap each other to prevent any boundary gap due to displacement of plates. The trapping processing is not executed unconditionally, and it is a common practice to decide whether or not to apply the processing according to the densities of the objects whose boundaries contact each other. This is because when the density of at least one of the objects whose boundaries contact each other is low, a boundary gap is not readily conspicuous if it is generated, and when the densities of both the objects are high, a boundary gap tends to be conspicuous. In addition, a technique for designating trapping for each page (for example, see Japanese Patent Laid-Open No. 2003-87548), and a technique for changing application conditions depending on attributes of objects (for example, see Japanese Patent Laid-Open No. 2004-237584) have been proposed.
The edge smoothing processing is executed to smooth jaggies of the edge (step-like edge) due to a screen structure visualized by dithering. In the edge smoothing processing, pixels are interpolated with reference to, for example, an original object so as to smoothly connect pixels of the edge of a target object after dithering. Since the edge smoothing processing is also executed for the density at which the screen structure tends to be conspicuous, whether or not to apply the processing is determined based on edge information of an object and, especially, density information of an edge portion. For example, when a density ranges from 0 to 255, since a screen structure due to dithering is conspicuous in a halftone object having a density=125, edge smoothing is applied to such object. By contrast, in case of an object whose density is low (e.g., a density=50), if the edge smoothing processing is applied after the dithering, the object is rimmed by the edge smoothing processing, and the density of that portion rises. In particular, when the original density is low, that density rise becomes conspicuous. Also, the jaggies of the edge portion of an object originally having a low density are not so conspicuous. Therefore, it is desirable not to apply edge smoothing to an object having a low density. For this reason, whether or not to apply edge smoothing is determined based on the density information of an object as in trapping (for example, see Japanese Patent No. 4137067).
In this way, both the trapping and edge smoothing processes are applied to enhance the image quality. However, when these processes are used together, various adverse effects occur. For example, by applying the trapping processing, the edges of objects whose boundaries contact each other shift to intrude each other's edges, thus overlapping the objects. This result is inevitably generated by the trapping processing. When the edge smoothing processing is applied to these objects, the edges of the two objects whose boundaries contact each other undergo the edge smoothing processing. For this reason, double edges appear, and these are further emphasized by the edge smoothing processing. Even if the type of a target object of the edge smoothing processing is limited to a character, the edge portion inevitably deteriorates. FIG. 10 shows this example. A character object 1001 and background object 1002 overlap each other near their boundaries as a result of the trapping processing. Then, the edge smoothing processing interpolates pixels 1003 at the edge of the character object that has undergone dithering. In this manner, the edges of regions to which both the trapping and edge smoothing processes are applied are especially excessively emphasized, resulting in considerable image deterioration.